1. Field of the Invention
This invention relates to an image exposure apparatus which can focus an image of an original onto a surface to be exposed by changing magnification, and more particularly, to an image exposure apparatus which includes an optical system including a single-focus lens unit and a mirror that moves when switching the length of an optical path.
2. Description of the Related Art
The following two types of apparatuses are typical image exposure apparatuses for focusing an image of an original onto a surface to be exposed by changing magnification. One is a zoom-lens-type image exposure apparatus which changes the distance among a plurality of lenses in accordance with magnification. This type of apparatus has a feature in that the length of the optical path between an original and a surface to be exposed can be maintained constant irrespective of magnification. Such a configuration is effective for reducing the size of the apparatus. Another is a mirror-zoom-type image exposure apparatus, in which a single-focus lens unit is moved in the direction of the optical axis in accordance with magnification, and a mirror is moved for switching the length of an optical path. Since the apparatus of this type requires a smaller number of lenses than the zoom-lens type apparatus, a decrease in the amount of light when light passes through the lens unit is smaller. Accordingly, a light source having a relatively small amount of light emission, such as a fluorescent lamp, can be used as a light source for illuminating an original, and therefore power consumption can be reduced.
FIG. 14 is a schematic diagram illustrating a magnification-varying mechanism of such a mirror-zoom-type image exposure apparatus.
In FIG. 14, a lens unit 35 having a single-focus lens, and a moving-mirror-mount unit 40 constituting a moving-mirror system are disposed at appropriate positions in the optical path of the apparatus. The lens unit 35 is driven by a motor 30 via a gear-wire system, comprising gears 31 and 32, a pulley 33, and a wire 34. That is, one end 34 a of the wire 34 provided via the gear 32 and the pulley 33 is connected to the upper left end of the lens unit 35, and another end 34b of the wire 34 is connected to the upper right end of the lens unit 35. Hence, the lens unit 35 can move in the direction of an arrow E in an enlargement mode, and in the direction of an arrow R in a reduction mode.
The driving force from the motor 30 is also transmitted to a gear 36 and to a cam 37 integrated therewith, and further to the mirror-mount unit 40 via link means 38 which is made in contact with the outer circumferential surface of the cam 37 by spring means 41. Thus, the lens unit 35 moves in the direction E when magnification is greater than 1, and in the direction R when magnification is smaller than 1. On the other hand, the mirror unit 40 moves to a left end position when magnification is 1, and moves toward the right in the directions E and R when magnification is greater than 1 and is smaller than 1, respectively.
In such a mirror-zoom-type image exposure apparatus, focus adjustment is performed by finely adjusting the relative position between the lens unit 35 and the mirror-mount unit 40 using a connection unit 39 for connecting the link means 38 to the mirror-mount unit 40.
Magnification adjustment is performed as follows.
As can be understood from FIG. 14, a lens barrel 42 disposed within the lens unit 35 is first adjusted in a direction of the optical axis (right or left in FIG. 14), and is then fixed at an appropriate position using an adjusting screw 43a which can freely move within a slit 42a.
The cam 37 for moving the mirror-mount unit 40 in a direction of the optical axis in synchronization with the movement of the lens unit 35 is fixed by causing the distal-end portion of the link means 38 to coincide with initial-position reference (for example, a scratched line, a projection or the like) provided in advance on the cam 37. The initial-position reference is provided, for example, at such a position that an image on an imaging surface has unit magnification with respect to an image on an original. The operation of the coincidence is performed by loosening an adjusting screw 37b fitted in a slit 37a provided in the cam 37, slightly rotating the cam 37, and tightening the adjusting screw 37b at an appropriate position.
The positions of the lens unit 35, the mirror unit 40 and the cam 37 are fixed in the above-described manner so that the image on the imaging surface and the image on the original are in the relationship of unit magnification. At that time, if optical characteristics of the lens unit 35 and the shape of the cam 37, and the like coincide with respective designed values, varying magnification coincides with a designed value even if the lens unit 35 and the mirror unit 40 are moved to the position of the maximum or minimum magnification.
Actually, however, the focal length of the lens unit 35 and the shape of the cam 37 have production tolerance, and therefore varying magnification which coincides with a designed value, and an expected state of focus on the imaging surface are not always obtained. As for focus on the imaging surface, if accuracy in components is within the range of tolerance, a focused state is obtained with accuracy within a provided range due to allowance in the depth of focus. However, as for varying magnification, even if accuracy in components is within the range of tolerance, the difference between a designed value and an actual value is relatively large, as shown in FIG. 5. The difference is particularly influenced by tolerance in the focal length of the lens unit. FIG. 5 illustrates measured values of magnification when seven kinds of lens units having focal lengths of 0.97-1.03 are used. As shown in FIG. 5, even if the positions of the lens unit 35 and the mirror unit 40 are adjusted in a state of unit magnification, actual values of magnification greatly deviate from designed values at the minimum and maximum values of magnification. This indicates that even if the operator sets a value of magnification, the actual value of magnification of an output image differs from the set value. If it is intended to increase accuracy in components is in order to solve such a problem, the production cost will increase.